ES2769249T3 - Organic photovoltaic cells using an ultrathin sensitization layer - Google Patents

Abstract

A photosensitive device comprising: a first electrode and a second electrode; a series of organic photoactive layers arranged between the first electrode and the second electrode, each layer of the series being in direct contact with the next layer in the series, the series of organic photoactive layers being arranged to form at least one donor-receptor heterojunction , the series comprising: a first organic photoactive layer (152) comprising a first host material serving as a donor; a second organic photoactive layer (980) comprising a second host material disposed between the first organic photoactive layer (152) and a third organic photoactive layer (154); and the third organic photoactive layer (154) comprising a third host material serving as an acceptor; in which the first host material, the second host material and the third host material are different, in which the second organic photoactive layer serves as an acceptor relative to the first organic photoactive layer or as a relative donor to the third photoactive layer, and a The distance from any point within the second organic photoactive layer host material to a boundary of that layer is nothing more than an exciton diffusion length over most of the area of the second organic photoactive layer, and characterized in that the second organic photoactive layer (980) is a unitary layer having openings (1001) through it, the first organic photoactive layer (152) being in direct contact with the third organic photoactive layer (154) through said openings .

Description

DESCRIPTION

Organic photovoltaic cells using an ultrathin sensitization layer

Field of the Invention

The present invention relates generally to organic photosensitive optoelectronic devices. More specifically, it is targeted at organic photosensitive optoelectronic devices that include an ultra-thin, low-mobility active layer that responds to near-infrared.

Background

Optoelectronic devices rely on the optical and electronic properties of materials to produce or detect electromagnetic radiation electronically or to generate electricity from ambient electromagnetic radiation.

Photosensitive optoelectronic devices convert electromagnetic radiation into an electrical signal or electricity. Solar cells, also called photovoltaic ("PV") devices, are a type of photosensitive optoelectronic device that is used specifically to generate electrical energy. Photoconductor cells are a type of photosensitive optoelectronic device that is used in conjunction with a signal detection circuit that monitors the resistance of the device to detect changes due to absorbed light. Photodetectors, which can receive an applied bias voltage, are a type of photosensitive optoelectronic device that are used in conjunction with current detection circuits that measure the current generated when the photodetector is exposed to electromagnetic radiation.

These three classes of photosensitive optoelectronic devices can be distinguished according to whether there is a rectifying junction as defined below and also according to whether the device operates with an applied external voltage, also known as bias or bias voltage. A photoconductor cell does not have a rectifier junction and normally operates with a polarization. A PV device has at least one rectifying joint and operates without polarization. A photodetector has at least one rectifying junction and is generally, but not always, polarized operated.

As used herein, the term "rectify" indicates, inter alia, that an interface has an asymmetric conduction characteristic, that is, the interface supports electronic load transport preferably in one direction. The term "semiconductor" indicates materials that can conduct electricity when charge carriers are induced by electromagnetic or thermal excitation. The term "photoconductor" generally refers to the process in which electromagnetic radiant energy is absorbed and thereby converted into excitation energy by carriers of electrical charge such that carriers can conduct (ie, transport) electric charge on a material. The term "photoconductive material" refers to semiconductor materials that are used for their property of absorbing electromagnetic radiation to generate carriers of electrical charge. As used herein, "upper" means further from the substrate, while "lower" means closer to the substrate. There may be intermediate layers, unless the first layer is specified to be "in physical contact" with the second layer.

When electromagnetic radiation of appropriate energy strikes an organic semiconductor material, a photon can be absorbed to produce an excited molecular state. In organic photoconductive materials, the generated molecular state is generally believed to be an "exciton," that is, a pair of electron holes in a bonded state that are transported as a quasi-particle. An exciton can have an appreciable lifetime before geminated recombination ("cooling"), which refers to the original electron and the hole that recombine with each other (as opposed to recombination with holes or electrons from other pairs). To produce a photocurrent, the electron holes that form the exciton are generally separated at a rectifying junction.

In the case of photosensitive devices, the rectifying junction is called the photovoltaic hetero junction. Types of organic photovoltaic heterojunctions include a donor-acceptor heterojunction formed at an interface of a donor material and an acceptor material, and a Schottky barrier heterojunction formed at the interface of a photoconductive material and a metal.

Figure 1 is an energy level diagram illustrating an example of a donor-receptor heterojunction. In the context of organic materials, the terms "donor" and "acceptor" refer to the relative positions of the energy levels of the highest occupied molecular orbital ("HOMO") and the lowest unoccupied molecular orbital ("LUMO"). ") of two organic materials in contact but different. If the LUMO energy level of one material in contact with another is low, then that material is an acceptor. Otherwise, it is a donor. It is energetically favorable, in the absence of an external polarization, that the electrons in a donor-acceptor junction move towards the acceptor material.

As used herein, a first HOMO or LUMO energy level is "greater than" or "higher than" a second HOMO or LUMO energy level if the first energy level is closer to the vacuum energy level 10. A higher HOMO energy level corresponds to an ionization potential ("IP") that has a smaller absolute energy relative to a vacuum level. Similarly, a higher LUMO energy level corresponds to an electronic affinity ("EA") that has a smaller absolute energy relative to the vacuum level. In a conventional energy level diagram, with the vacuum level at the top, the LUMO energy level of a material is higher than the HOMO energy level of the same material.

After absorption of a photon 6 in donor 152 or acceptor 154, an exciton 8 is created, exciton 8 dissociates at the rectifying interface. Donor 152 transports the gap (open circle) and acceptor 154 transports the electron (dark circle).

An important property in organic semiconductors is operator mobility. Mobility measures the ease with which a charge carrier can move through a conductive material in response to an electric field. In the context of organic photosensitive devices, a material that is preferably electron-conducting due to high electron mobility can be referred to as an electron transport material. A material that conducts preferably through voids due to high mobility of voids can be referred to as a void transport material. A layer that is electrically conductive, due to mobility and / or position in the device, may be referred to as an electron transport layer ("ETL"). A layer that preferably conducts through gaps, due to mobility and / or position in the device, may be referred to as a gap transport layer ("HTL"). Preferably, but not necessarily, an acceptor material is an electron transport material and a donor material is a gap transport material.

It is well known in the art how to pair two organic photoconductive materials to serve as a donor and acceptor in a photovoltaic heterojunction based on carrier mobility and relative levels of HOMO and LUMO, and is not addressed in this case.

A common feature of raw semiconductors, as well as insulators, is a "band gap". The band gap is the energy difference between the highest energy level filled with electrons and the lowest energy level that is empty. In an inorganic insulator or inorganic semiconductor, this energy difference is the difference between the edge of the valence band (top of the valence band) and the edge of the conduction band (bottom of the conduction band). In an organic semiconductor or organic insulator, this energy difference is the difference between HOMO and LUMO. The band gap of a pure material lacks energy states where electrons and holes can exist. The only carriers available for conduction are the electrons and holes that have enough energy to get excited through the band gap. In general, semiconductors have a relatively small band gap compared to insulators.

In terms of an energy band model for organic semiconductors, only the electrons on the LUMO side of the band gap are charge carriers, and only the holes on the HOMO side of the band gap are charge carriers.

As used herein, the term "organic" includes polymeric materials as well as small molecule organic materials that can be used to make organic optoelectronic devices. "Small molecule" refers to any organic material other than a polymer, and "small molecules" can actually be quite large. Small molecules can include repeating units in some circumstances. For example, the use of a long chain alkyl group as a substituent does not remove a molecule of the "small molecule" class. Small molecules can also be incorporated into polymers, for example, as a pendant group in a main polymer segment or as part of the main segment. Small molecules can also serve as the core fraction of a dendrimer, which consists of a series of chemical layers built on top of the rest of the nucleus. The core fraction of a dendrimer can be a fluorescent or phosphorescent small molecule emitter. A dendrimer can be a "small molecule". In general, a small molecule has a defined chemical formula with a molecular weight that is the same from molecule to molecule, while a polymer has a defined chemical formula with a molecular weight that can vary from molecule to molecule. As used herein, "organic" includes hydrocarbyl metal complexes and hetero atom-substituted hydrocarbyl ligands.

For a further background explanation and a description of the state of the art of organic photosensitive devices, including their general construction, characteristics, materials and functions, the United States Patent No. 6,657,378 to Forrest et al. , US Patent No. 6,580,027 to Forrest et al., and US Patent No. 6,352,777 to Bulovic et al.

Summary of the invention

The invention relates to a photosensitive device according to claim 1.

A photosensitive device comprises a series of organic photoactive layers arranged between a first electrode and a second electrode. Each layer in the series is in direct contact with the next layer in the series. The series of organic photoactive layers is arranged to form at least one donor-acceptor heterojunction. The series of organic photoactive layers includes a first organic photoactive layer comprising a first host material serving as a donor, a second organic photoactive layer comprising a second host material disposed between the first organic photoactive layer and a third organic photoactive layer, and the organic third photoactive layer comprising a third host material that serves as an acceptor. The first host material, the second host material, and the third host material are different. The second organic photoactive layer serves as an acceptor relative to the first organic photoactive layer or as a donor relative to the third photoactive layer. A distance from any point within the second host material of the second organic photoactive layer to a boundary of that layer is no more than an exciton diffusion length over most of the area of the second organic photoactive layer. Preferably, the distance from any point within the second host material of the second organic photoactive layer to a boundary of that layer is no more than an exciton diffusion length over the entire area of the second organic photoactive layer.

The second organic photoactive layer is a unitary layer having openings therethrough, the first organic photoactive layer being in direct contact with the third organic photoactive layer through said openings.

The second host material is preferably a small molecule. The first host material and said third host material are also preferably small molecules. The second small molecule host material preferably has an absorption coefficient of at least 5 x 104 cm-1 across a wavelength band of 600nm to 900nm.

Each of the first organic photoactive layer, the second organic photoactive layer, and the third organic photoactive layer may have different absorption spectra.

The second organic photoactive layer can serve as a donor, with the first host material and the second host material having different absorption spectra. Serving as a donor, e1HOMO of the second host material is preferably no more than 0.16 eV above a HOMO of the first host material, and the band gap of the second host material is preferably less than a band gap of the first host material. If serving as a donor, the second host material may have a gap mobility of less than 1 x 10-9 cm2 / Vs and an absorption coefficient of at least 5 x 104 cm-1 across a wavelength band from 600 nm to 900 nm. Examples of the second host material to serve as a donor include tin (II) phthalocyanine (SnPc) and lead phthalocyanine (PbPc). An example of a third host material for the third host material to serve as acceptor with the SnPc or PbPc donor is C60.

The second organic photoactive layer can serve as an acceptor, with the second host material and the third host material having different absorption spectra. Serving as an acceptor, the LUMO of the second host material is preferably not more than 0.16 eV below a LUMO of the third host material. If it serves as an acceptor, the band gap of the second host material is preferably less than a band gap of the third host material.

The thickness of the second organic photoconductive layer is preferably not more than 200 A, or more preferably, not more than 100 A.

The second host material preferably has an absorption coefficient of at least 5 x 104 cm-1 across a wavelength band of 600nm to 900nm, and in which at least a portion of the second organic photoactive layer is arranged at an optical path length of Ar d A1 / 4 from a reflective surface of the device, where A1 is a wavelength in the wavelength band of 600nm to 900nm, d is an integer> 0, and the reflective surface is reflective for at least 50% of the incident light at A1. The reflective surface can be provided, for example, by one of the first electrode, the second electrode and a reflector.

The device may comprise a stack of photoactive cells arranged between the first electrode and the second electrode. In such a stack, the series of organic photoactive layers can be part of a first cell in the photoactive cell stack. The stack also includes at least one second photoactive cell comprising another donor-acceptor heterojunction. The first cell and the second cell in the stack may have different absorption characteristics. Preferably, an average absorption of the first cell is greater than the average absorption of the second cell in a wavelength range A1 ± 5%, and an average absorption of the second cell is greater than the average absorption of the first cell in a range of wavelengths A2 ± 5%, where A1> A2 10%.

The series of organic photoactive layers may further comprise a mixed or crude donor-acceptor layer disposed between the first organic photoactive layer and the third organic photoactive layer. The mixed or raw donor-acceptor layer includes both the first host material from the first organic photoactive layer and the third host material from the third organic photoactive layer.

Brief description of the drawings

Figure 1 is an energy level diagram illustrating a donor-accept heterojunction.

Figure 2 illustrates an organic photosensitive device that includes a donor-accept heterojunction.

Figure 3 illustrates a donor-accept bilayer that forms a flat heterojunction.

Figure 4 illustrates a hybrid heterojunction including a mixed heterojunction between a donor layer and an acceptor layer.

Figure 5 illustrates a raw heterojunction.

Figure 6 illustrates an organic photosensitive device that includes a Schottky barrier heterojunction.

Figure 7 illustrates series tandem photosensitive cells.

Figure 8 illustrates tandem photosensitive cells in parallel.

Figure 9 illustrates a planar heterojunction modified to include an additional thin photoactive layer.

Figure 10A illustrates a cross section of an example of the thin photoactive layer.

Figure 10B illustrates a cross section of an inventive example of the thin protective layer.

Figure 11 illustrates an energy level diagram that includes the thin photoactive layer as a donor.

Figure 12 illustrates an energy level diagram that includes the thin photoactive layer as an acceptor.

Figure 13 illustrates a hybrid heterojunction including the thin photoactive layer as a donor.

Figure 14 illustrates a hybrid heterojunction including the thin photoactive layer as an acceptor.

Figure 15 illustrates a flat joint including a plurality of thin photoactive layers.

Figure 16 illustrates an energy level diagram including the plurality of thin photoactive layers for flat bonding in Figure 15.

Figure 17 illustrates the placement of the various layers to maximize absorption by the thin photoactive layer. Figure 18 illustrates the current density as a function of voltage, in the dark and under various illumination intensities, of an example of a photosensitive cell that includes the thin photoactive layer.

Figure 19 illustrates the cell fill factor (FF) and open circuit voltage (V ^oc ) as a function of illumination intensity for the same device as Figure 14.

Figure 20 illustrates the power conversion efficiency versus lighting intensity for the same device as Figure 14.

Figure 21 illustrates the external quantum efficiency and absorption coefficients as a functional wavelength for the same device as Figure 14.

Figure 22 illustrates the X-ray diffraction results for the purified SnPc source material used for the thin photoactive layer, both powdered and deposited in an ITO.

Figure 23 illustrates the surface morphology of a 250A thick SnPc film on quartz.

The figures are not necessarily drawn to scale.

Detailed description

An organic photosensitive device comprises at least one photoactive region in which light is absorbed to form an exciton, which can subsequently dissociate into an electron and a hole. Figure 2 shows an example of an organic photosensitive optoelectronic device 100 in which the photoactive region 150 comprises a donor-acceptor heterojunction. The "photoactive region" is a part of a photosensitive device that absorbs electromagnetic radiation to generate excitons that can dissociate in order to generate an electric current. Device 100 comprises an anode 120, an anode smoothing layer 122, a donor 152, an acceptor 154, an exciton blocking layer ("EBL") 156 and a cathode 170, on a substrate 110.

Examples of EBL 156 are described in United States Patent No. 6,451,415 to Forrest et al., Which is cited for its disclosure related to EBLs. An additional explanation of EBLs can also be found in Peumans et al., "Efficient photon harvesting at high optical intensities in ultrathin organic double-heterostructure photovoltaic diodes", Applied Physical Letters 76, 2650-52 (2000). EBLs reduce cooling by preventing excitons from migrating out of donor and / or acceptor materials.

The terms "electrode" and "contactor" are used interchangeably herein to refer to a layer that provides a means of supplying photogenerated current to an external circuit or providing a bias current or voltage to the device. As illustrated in Figure 2, anode 120 and cathode 170 are examples. The electrodes can be composed of metals or "metal substitutes". In this case, the term "metal" is used to encompass both materials composed of an elementally pure metal, as well as metal alloys that are materials composed of two or more elementally pure metals. The term "metal substitute" refers to a material that is not a metal within the normal definition, but has properties similar to those of metals, such as conductivity, such as wideband gap doped semiconductors, degenerate semiconductors, conductive oxides, and conductive polymers. The electrodes can comprise a single layer or multiple layers (a "composite" electrode), they can be transparent, semi-transparent or opaque. Examples of electrodes and electrode materials include those described in US Patent No. 6,352,777 to Bulovic et al., And US Patent No. 6,420,031 to Parthasarathy, et al.

As used herein, a layer is said to be "transparent" if it transmits at least 50% of ambient electromagnetic radiation at a relevant wavelength.

Substrate 110 can be any suitable substrate that provides the desired structural properties. The substrate can be flexible or rigid, flat or non-flat. The substrate can be transparent, translucent or opaque. Rigid plastics and glass are examples of preferred rigid substrate materials. Flexible plastics and sheet metal are examples of preferred flexible substrate materials.

An anode smoothing layer 122 may be located between the anode layer 120 and the donor layer 152. The anode smoothing layers are described in US Patent 6,657,378 to Forrest et al.

In Figure 2, the photoactive region 150 comprises donor material 152 and acceptor material 154. Organic materials for use in the photoactive region may include organometallic compounds, including cyclometalated organometallic compounds. The term "organometallic", as used herein, is generally understood by one of skill in the art and as given, for example, in Chapter 13 of "Inorganic Chemistry" (2nd Edition) by Gary L. Miessler and Donald A. Tarr, Prentice Hall (1999).

Organic layers can be manufactured using vacuum deposition, rotational coating, organic vapor phase deposition, ink jet printing, and other methods known in the art.

Examples of various types of donor-acceptor hetero junctions are shown in Figures 3-5. Figure 3 illustrates a donor-acceptor bilayer that forms a flat heterojunction. Figure 4 illustrates a hybrid heterojunction including a mixed heterojunction 153 comprising a mixture of donor and acceptor materials. Figure 5 illustrates an idealized "raw" heterojunction. A raw heterojunction 253, ideally the photocurrent, has a single continuous interface between donor material 252 and acceptor material 254, although there are typically multiple interfaces on actual devices. Mixed and raw hetero junctions can have multiple donor-acceptor interfaces as a result of having plural material domains. Domains that are surrounded by material of the opposite type (eg, a domain of donor material surrounded by recipient material) may be electrically isolated, such that these domains do not contribute to the photocurrent. Other domains may be connected by percolation paths (direct photocurrent paths), such that these other domains may contribute to the photocurrent. The distinction between a mixed and a raw heterojunction lies in the degrees of phase separation between the donor and acceptor materials. In a mixed heterojunction, there is little or no phase separation (the domains are very small, for example, less than a few nanometers), while in a crude heterojunction, there is significant phase separation (for example, forming domains with sizes from a few nanometers to 100 nm).

Mixed small molecule heterojunctions can be formed, for example, by joint deposition of donor and acceptor materials using vacuum deposition or vapor deposition. Small molecule crude heterojunctions can be formed, for example, by controlled growth, co-deposition with post-annealing, or solution processing. Raw or mixed polymeric heterojunctions can be formed, for example, by solution processing of polymer blends of donor and acceptor materials.

If a photoactive region includes a mixed layer (153) or raw layers (252, 254) and one or both donor (152) and acceptor layers (154), the photoactive region is said to include a "hybrid" heterojunction. The arrangement of the layers in Figure 4 is an example. For further explanation of hybrid heterojunctions, US patent application 2005-0224113 A1 is cited entitled "High efficiency organic photovoltaic cells employing hybridized mixed-planar heterojunctions" by Jiangeng Xue et al., Published Oct. 13. from 2005.

In general, flat heterojunctions have good carrier conduction, but poor exciton dissociation; a mixed layer has poor carrier conduction and good exciton dissociation, and a crude heterojunction has good carrier conduction and good exciton dissociation, but may experience charge build-up at the end of the cul-de-sacs material ", decreasing efficiency. Unless otherwise indicated, flat, mixed, crude, and hybrid heterojunks may be used interchangeably as donor-acceptor heterojunctions in all of the embodiments described herein.

Figure 6 shows an example of an organic photosensitive optoelectronic device 300 in which the photoactive region 350 is part of a Schottky barrier heterojunction. Device 300 comprises a transparent contact 320, a photoactive region 350 comprising an organic photoconductive material 358, and a Schottky 370 contact. The Schottky 370 contact is normally formed as a metal layer. If the photoconductive layer 358 is an ETL, a high working function metal such as gold can be used, while if the photoconductive layer is an HTL, a low working function metal such as aluminum, magnesium or indium can be used. In a Schottky barrier cell, a built-in electric field associated with the Schottky barrier separates the electron and the hole in an exciton. In general, this field-assisted exciton dissociation is not as effective as dissociation at a donor-acceptor interface.

Devices as illustrated may be connected to an element 190. If the device is a photovoltaic device, element 190 is a resistive load that consumes or stores energy. If the device is a photodetector, element 190 is a current detection circuit that measures the current generated when the Photodetector is exposed to light, and may polarize the device (as described, for example, in United States published patent application 2005-0110007 A1, published May 26, 2005 to Forrest et al.) . If the rectifying junction is removed from the device (for example, using a single photoconductive material as the photoactive region), the resulting structures can be used as a photoconductive cell, in which case element 190 is a signal detection circuit to monitor changes in resistance through the device due to light absorption. Unless otherwise indicated, each of these arrangements and modifications may be used for the devices in each of the drawings and embodiments disclosed herein.

An organic photosensitive optoelectronic device may also comprise transparent charge transfer layers, electrodes, or charge recombination zones. A charge transfer layer may be organic or inorganic, and may or may not be photoconductively active. A charge transfer layer is similar to an electrode, but does not have an external electrical connection to the device and delivers charge carriers from one subsection of an optoelectronic device to the adjacent subsection. A charge recombination zone is similar to a charge transfer layer, but allows the recombination of electrons and holes between adjacent subsections of an optoelectronic device. A loading recombination zone may include semi-transparent metal or metal substitute recombination centers comprising nanogroups, nanoparticles, and / or nanobars, as described, for example, in US Patent No. 6,657,378 to Forrest et al. to the.; United States patent application 10 / 915,410 entitled "Organic Photosensitive Devices" by Rand et al. filed on August 11, 2004; and US Patent Application 10 / 979,145 titled "Stacked Organic Photosensitive Devices" by Forrest et al. filed on November 3, 2004; each cited for disclosure of materials and structures of recombination sites. A charge recombination zone may or may not include a transparent matrix layer in which the recombination centers are embedded. A charge transfer layer, electrode, or charge recombination zone can serve as a cathode and / or anode for subsections of the optoelectronic device. An electrode or charge transfer layer can serve as a Schottky contact.

Figures 7 and 8 illustrate examples of tandem devices including such transparent charge transfer layers, electrodes, and charge recombination zones. In device 400 in FIG. 7, photoactive regions 150 and 150 'are electrically stacked in series with an intervention lead region 460. As illustrated without external electrical connections, the intervention lead region 460 may be a recombination zone of charge or it can be a charge transfer layer. As the recombination zone, region 460 comprises recombination centers 461 with or without a transparent matrix layer. If there is no matrix layer, the arrangement of the zone-forming material may not be continuous through region 460. Device 500 in Figure 8 illustrates the photoactive regions 150 and 150 'stacked electrically in parallel, the cell being superior in an inverted configuration (i.e. descending cathode). In each of Figures 7 and 8, the photoactive regions 150 and 150 'and the blocking layers 156 and 156' may be formed from the same respective materials, or different materials, depending on the application. Also, the photoactive regions 150 and 150 'may be of the same type (ie, flat, mixed, raw, hybrid) of heterojunction, or may be of different types.

In each of the devices described above, layers can be omitted, such as exciton blocking layers. Other layers can be added, such as reflective layers or additional photoactive regions. The order of the layers can be altered or reversed. A concentrator or entrapment configuration can be used to increase efficiency, as disclosed, for example, in Forrest et al., US Patent No. 6,333,458. and United States Patent No. 6,440,769 to Peumans et al.

The coatings can be used to focus optical energy on the desired regions of a device, as described, for example, in US Patent Application No. 10 / 857,747 entitled "Aperiodic dielectric multilayer stack" by Peumans et al. filed June 1, 2004. In tandem devices, transparent insulating layers can form between the cells, with the electrical connection between the cells through electrodes. Also in tandem devices, one or more of the photoactive regions may be a Schottky barrier heterojunction rather than a donor-acceptor heterojunction. Provisions other than those specifically described may be used.

Photovoltaic devices produce a photogenerated current when connected via a load and irradiated by light. When irradiated under infinite load, a photovoltaic device generates its maximum possible voltage, open circuit V or V ^oc . When irradiated with its electrical contacts short circuited, a photovoltaic device generates its maximum possible current, short circuit I, or I ^sc . When actually used to generate power, a photovoltaic device is connected to a finite resistive load, and the power output is provided by the product of the current and voltage, I * V. The maximum total power generated by a photovoltaic device is inherently unable to beat the product, I ^sc * V ^oc . When the load value is optimized for maximum power extraction, the current and voltage have the values, Imax and Vmax, respectively.

A figure of merit for photovoltaic devices is the fill factor, ff defined as:

# - í i m á x V max} / {; i s c ^ o c}

where ff is always less than 1, as I ^sc and V ^{or c are} never obtained simultaneously during actual operation. However, as ff approaches 1, the device has less internal or serial resistance, and therefore offers a higher percentage of the product of I ^sc and V ^oc at load under optimal conditions. Where Pinc is the power incident on a device, the power efficiency of the device, p, can be calculated by:

F = # * CSC * V0C) / Bine

Organic photovoltaic cells have many potential advantages compared to traditional silicon-based devices. Organic photovoltaic cells are light in weight, inexpensive in the use of materials, and can deposit on low-cost substrates, such as flexible plastic sheeting. However, some organic PV devices typically have a relatively low external quantum efficiency, on the order of 1% or less. This, in part, is believed to be due to the second-order nature of the intrinsic photoconductive process. That is, the generation of carriers requires the generation, diffusion, and ionization or collection of excitons. There is an efficacy associated with each of these processes. The subscripts can be used as follows: P for power efficiency, EQE for external quantum efficiency, A for photon absorption, ED for exciton diffusion, CC for charge collection, and INT for internal quantum efficiency. Using this notation:

^ = * Hs

V ^p V EQE and A ?? ED V cc

- *

í / eqe V ^a í? int

Photodetectors and photovoltaic (PV) cells using small molecular weight organic thin films deposited on plastic substrates have the potential advantages of being low cost and lightweight. See S.R. Forrest, "The path to ubiquitous and low-cost organic electronic appliances on plastic", Nature 428, 911-918 (2004). Recently, the power conversion efficiency of molecular organic PV cells has been constantly improved due to the use of new materials and device architectures. See S.R. Forrest, "The Limits to Organic Photovoltaic Cell Efficiency", MRS Bulletin 30, 28-32 (2005); and J. Xue et al., "Asymmetric tandem organic photovoltaic cells with hybrid planar-mixed molecular heterojunctions", Applied Physical Letters 85, 5757-5759 (2004).

However, one problem with organic PV energy conversion is the limited overlap between active layer absorption with the solar spectrum. In fact, over 60% of the total solar photon flux is at wavelengths A> 600 nm with approximately 50% in the red and near infrared (NIR) spectrum at 600 <A <1000 nm. Therefore, new materials must be developed that can absorb NIR radiation and effectively convert absorbed photons into current.

Recently, a solar cell based on NIR radiation sensitive polymers up to A = 1000nm achieved a power conversion efficiency of 0.3% under 1 sunlight. See X.J. Wang et al., Applied Physical Letters 85, 5081 (2004). However, previous efforts with small molecule materials have failed to extend the device's responsiveness to NIR.

Research into the development of a small molecule photosensitive device has produced a new architecture that, in addition to achieving NIR responsiveness, can be used to adjust and / or extend the sensitivity of any donor-acceptor heterojunction. By incorporating a thin sensitizing layer between the donor and acceptor, this new architecture enables the construction of photosensitive devices that retain the full thickness of the device, but can produce photocurrent in a part of the spectrum that would otherwise be inaccessible using other materials. .

In addition, a wider range of different materials are available for use as a thin sensitization layer that would otherwise be viable as a conventional donor or acceptor layer. For example, a material that has low charge carrier mobility can be used without loss of device performance. By pairing the thin sensitization layer with an energy-friendly donor or acceptor layer, the thin sensitization layer contributes to the photocurrent, without preventing carrier transport between the donor and acceptor.

Figure 9 illustrates a donor-acceptor heterojunction, as used in the devices (eg, 100, 400, 500) discussed above, modified to incorporate the new thin sensitization layer 980 within a photoactive region 950. The sensitization layer Thin 980 comprises a host material that is different from donor 152 or acceptor 154. A "host material" of an organic photoactive layer, as defined herein, is a photoactive organic molecule that constitutes more than 50 mole% of the layer respective. Thin sensitization layer 980 can be configured as either a donor or an acceptor. In any case, a distance from any point within the host material of the sensitization layer 980 to a boundary of that layer is not more than one exciton diffusion length over a greater part (> 50%) of the zone of the layer 980 thin sensitization. An exciton diffusion length is the displacement distance at which excitons 1 / e deteriorate / se recombine. The distance is in a "larger part" of the area since layer 980 may include edge effects and small variations in local thickness. (eg, bumps and beads). Preferably, edge effects and the like are avoided, and the distance from any point within the host material of the thin sensitization layer 980 to a boundary of that layer is no more than an exciton diffusion length over the entire area. of the thin sensitization layer 980. Preferably, even if twice this distance through the sensitization layer 980 is greater than 200 A, the thickness of the layer is not greater than 200 A. More preferably, the thickness of the layer is not greater than 100 A.

Due to efficiency issues, a balance can be struck between the thinness and the photocurrent contribution of sensitization layer 980. Favoring a thin layer is the desire to minimize resistivity across the photoactive layer for carriers transiting between the donor layer 152 and acceptor layer 154, and maximize the ability of excitons formed in sensitization layer 980 to reach a rectification interface. Favoring a thick layer is the volume at which photons can be absorbed in the wavelength absorption band of sensitization layer 980. While two exciton diffusion lengths is an idealized upper limit of thickness, it is expected that for For most combinations of materials and light source spectra, a thinner sensitizing layer 980 is desirable.

While the thin sensitizing layer 980 may be solid, the inventive thin sensitizing layer 980 of the invention is porous, as illustrated in Figure 10A. If porous, sensitization layer 980 includes a plurality of paths 1001 (ie, openings) through a unitary layer. Adjacent layers (eg, donor 152 and acceptor 154) contact each other directly through paths 1001, providing cargo carriers with a direct route between the layers. Pathways 1001 may manifest during the deposition process due, for example, to the use of a thin layer 980 such that there is a natural irregularity in coverage (for example, growth until material nucleation sites are connected, but before all gaps to be filled), or having irregularities on the surface of the underlying layer (eg donor 152) resulting in uneven coverage.

Figure 10B is another example of thin layer 980. In this example, which is useful in understanding the invention, the layer comprises a plurality of discontinuous islands of material. Islands 1002 can manifest during the deposition process due to, for example, detecting growth shortly after nucleation.

A solid layer, a discontinuous island layer, or a porous unitary layer can be selected during manufacturing simply by controlling how long growth is allowed to continue after nucleation of the material. While all three styles of layer 980 can be operated, the porous unit layer is preferred for its balance of photoactive surface area while providing direct charge carrier transit between adjacent layers (eg, donor 152 and acceptor 154) across the roads 1001.

Preferably, the thin sensitization layer 980 is a small molecule material. Similarly, it is preferred that donor layer 152 and acceptor layer 154 also be small molecule materials. To absorb a NIR, the second material can have an absorption coefficient of at least 5 * 104 cm-1 across a wavelength band of 600nm to 900nm. To maximize the coverage of the absorption spectra, the three photoactive layers (152, 980, 154) can each have a different absorption spectrum.

When the thin sensitization layer 980 serves as the donor in FIG. 11 it has a different host material than the donor layer 152. Preferably, the HOMO of the thin sensitization layer 980 is not more than 5kT above the HOMO of the donor layer 152 (k is the Boltzmann constant and T is the operating temperature), thus avoiding the capture of gaps at the interface of donor layer (152) -sensitization layer (980). This difference in HOMO levels is illustrated as AE1 in Figure 11.

Operating temperatures for organic photosensitive devices are commonly specified as having a range of T = -40 ° C to 100 ° C. Therefore, using 100 ° C as the maximum limit and solving for 35 kT (i.e. 5 x 1,3806505E-23 (J / K) / 1,602E-19 (J / eV) x (T ° C 273.15 ) ° K), the HOMO of the thin sensitization layer 980 should not be more than 0.16 eV above the HOMO of the donor layer 152.

If the sensitization layer 980 is arranged as a donor, the band gap of the material forming the sensitization layer 980 may be less than the band gap of the material forming the donor layer 152. Since the absorption sensitivity is, In general, inversely proportional to the band gap of a pure material, arranging the band gaps in this way allows for the absorption of photons of greater wavelength than would occur with donor layer 152 alone.

By maintaining the sensitization layer 980 thin, a whole class of materials can be used that would otherwise not be viable with reasonable efficiency in a photoactive device. For example, if sensitization layer 980 is used as a donor, a host material of sensitization layer 980 may have a gap mobility of less than 1 x 10-9 cm2 / Vs. Classically, the use of a material as a donor is counterintuitive, since the high mobility of the gaps is a feature sought by donors to maximize the external quantum efficiency of the device. However, there are many low void mobility materials, such as phthalocyanine from tin (II) (SnPc) and lead phthalocyanine (PbPc) with poor void mobility, but with absorption coefficients of at least 5 x 104 cm-1 across a wavelength band of 600 nm to 900 nm .

Therefore, materials with highly desirable absorption properties but with poor mobility properties can be used in sensitization layer 980, although such materials may not have been used previously due to their poor mobility properties.

When thin sensitizing layer 980 serves as an acceptor in FIG. 12, it has a different host material than acceptor layer 154. Preferably, the LUMO of thin sensitizing layer 980 is no more than 5kT below the LUMO of the layer. acceptor 154, thereby avoiding electron capture at the acceptor layer interface (154) - sensitization layer (980). This difference in LUMO levels is illustrated as AE2 in Figure 12.

If sensitization layer 980 is provided as an acceptor, the band gap of the material forming sensitization layer 980 may be less than the band gap of the material forming acceptor layer 154. Since the absorption sensitivity is, In general, inversely proportional to the band gap of a pure material, arranging the band gaps in this way allows absorption of longer wavelength photons than would occur with acceptor layer 154 alone.

By maintaining the sensitization layer 980 thin, a host material having an electron mobility of less than 1 x 10-9 cm2 / V can be used. Classically, the use of a material such as an acceptor is counterintuitive, since high electron mobility is a feature sought by acceptors to maximize the external quantum efficiency of the device. Therefore, materials with highly desirable absorption properties but with poor mobility properties can be used in sensitization layer 980, although such materials may not have been used previously due to their poor mobility properties.

The sensitization layer 980 can also be used in a hybrid heterojunction, as illustrated in Figures 13 and 14. In Figure 13, the sensitization layer 980 serves as a donor within the 1350 photoactive region. Preferably, as a donor, the HOMO The thin sensitization layer 980 is no more than 5kT above the HOMO of donor layer 152 (AE1). In FIG. 14, sensitization layer 980 serves as an acceptor within photoactive region 1450. Preferably, as an acceptor, the LUMO of thin sensitization layer 980 is no more than 5kT below the LUMO of acceptor layer 154 (AE2 ).

Figure 15 illustrates a photoactive region 1550 that includes a plurality of donor sensitization layers (980a, 980b) and a plurality of acceptor sensitization layers (980c, 980d). Figure 16 is an energy level diagram for a photosensitive cell comprising the plurality of thin sensitization layers 980a-d within the single photoactive region in Figure 15. Preferably, to avoid entrapment of the charge carrier: the HOMO of the first donor layer of thin sensitization 980a is not more than 5kT above 1HOMO of the donor layer 152 (AE1,1); The HOMO of the second sensitizing donor thin layer 980b is not more than 5kT above the HOMO of the first thin sensitization donor layer 980a (AE1,2); the LUMO of the first thin sensitizing layer 980c is not more than 5kT below the LUMO of the second thin sensitizing acceptor layer 980d (AE2,1); and the LUMO of the second thin sensitizing acceptor layer 980d is not more than 5kT below the LUMO of the acceptor layer 154 (AE2,2).

Photoactive regions (950, 1350, 1450, 1550) having the one or more sensitizing layers 980 can be interchanged with photoactive regions 150 and / or 150 'in the previously treated photosensitive devices, including devices 100, 400, and 500.

As illustrated in Figure 17, the performance of the device is enhanced by placing at least a portion of the thin sensitization layer 980 (as illustrated, arranged as a layer within a photoactive region 950/1350/1450/1550 on the device in tandem shown in Figure 7) at an optical path length of Ar D A1 / 4 (d is an integer> 0) of a reflective surface of the device. If, for example, sensitization layer 980 has an absorption coefficient of at least 5 x 104 cm-1 across a wavelength band of 600nm to 900nm, then A1 is a wavelength in the band wavelength from 600nm to 900nm. By placing a peak in this band (750nm, as illustrated) to overlay a part of the sensitization layer, it is better ensured that light will be absorbed in the desired bandwidth.

The reflective surface preferably reflects at least 50% of the incident light at A1. The reflective surface may be a separate layer, or may be provided by one of the electrodes (eg, cathode 170 in Figure 2 and Figure 7; anode 170 'in Figure 8).

If the sensitization layer 980 is in a photoactive region of a first cell within a cell stack, the overall performance can be improved by configuring the different cells to have different absorption characteristics. Preferably, the average absorption of the first cell (having sensitization layer 980) is greater than the average absorption of the second cell in a wavelength range A1 ± 5%, and an average absorption of the second cell is greater than the average absorption of the first cell in an interval of wavelengths Á2 ± 5%; where A1> Á2 10%, A1 is a wavelength in the absorption band of 600 nm to 900 nm, and the host material serving as the sensitization layer 980 has an absorption coefficient of at least 5 x 104 cm-1 through 600nm to 900nm.

Since the materials that constitute a donor and the materials that constitute an acceptor depend on the relative energy levels between layers, the same material (for example, SnPc, PbPc) can serve as a donor layer with a set of materials as a donor 152 / 252 and acceptor 154/254, and serve as an acceptor layer with a different set of materials as donor 152/252 and acceptor 154/254.

Examples

The experiments were performed in an organic photovoltaic cell based on a tin (II) phthalocyanine (SnPc) / C60 donor / acceptor heterojunction with sensitivity at wavelengths of A> 900 nm. The low void mobility in SnPc polycrystalline thin films, jh = (2 ± 1) * 10-10 cm2 / Vs, avoids the use of thick layers, leading to low fill factors and low power conversion efficiencies. However, due to its high absorption coefficient, a 50A thick SnPc layer produces external quantum efficiencies of external solar cells of up to 21% at A = 750nm. With the double indium-tin-oxide / 100A copper phthalocyanine / 50A SnPc / 540A C60 / 75A batocuproin / Ag heterostructure, an energy conversion efficiency of (1.0 ± 0.1)% is obtained under solar lighting AM1.5G conventional 1 sol and efficiencies of (1.3 ± 0.1)% under intense sunlight AM1.5G conventional (10 soles). For an example of a device without the SnPc sensitization layer (an indium tin oxide / copper phthalocyanine / C60 / Ag Device), see "4.2% efficient organic photovoltaic cells with low series resistances", by J. Xue et al., Applied physical letters 84, 3013-3015 (2004).

The photogeneration process in organic materials begins with the absorption of a photon that creates a bound exciton or electron-hole pair. In effective organic photovoltaic devices, the exciton dissociates at a donor-acceptor interface into free charge carriers, which are subsequently collected at their respective electrodes. Using a tin (II) phthalocyanine (SnPc) layer as an electron donor in double heterostructure organic solar cells, the SnPc layer efficiently converts incident NIR light to photocurrent. Power conversion efficiencies of r¡p = (1.0 ± 0.1)% are possible under conventional 1-sol AM1.5G solar illumination (1.5 global air mass). The highly efficient results since the fill factor is FF = 0.5 for ultra-thin SnPc layers, whereas the previously demonstrated devices (XJ Wang et al., Applied Physical Letters 85, 5081 (2004)) had FF = 0 , 32. This FF height is achieved by controlling the thickness of the SnPc layer, so that its low void mobility does not adversely affect the performance of the device.

All devices were manufactured in 1500 A thick Indium Tin Oxide (ITO) layers commercially precoated on glass substrates with a sheet resistance of 15 Q / s. The solvent-cleaned ITO surface (see J. Xue et al., Applied Physical Review 95, 1869 (2004)) was UV / O3 treated for 5 minutes immediately prior to loading the substrates into a high vacuum chamber (base pressure ~ 3 * 10.7 Torr), where organic layers and a 1000 A thick cathode of Ag were deposited by means of thermal evaporation. Before deposition, organic materials were purified in three cycles using thermal gradient sublimation under vacuum. (See R.A. Laudise et al., Journal of Crystal Growth 187, 449 (1998)).

The Ag cathode was evaporated through a shadow mask with 1 mm diameter openings to define the area of the device. Current density versus voltage ( JV) characteristics were measured in the dark and under simulated AM1.5G solar illumination (Oriel Instruments) using an HP4155B semiconductor parameter analyzer. Measurements of illumination intensity and quantum efficiency were made using conventional methods employing a traceable calibrated Si detector from the National Renewable Energy Laboratory (ASTM Standards E1021, E948, E973 American Society for Testing and Materials, West Conshohocken PA).

Differences between simulated and actual (i.e. conventional) solar spectra were corrected in the final determination of efficacy. Absorption spectra were measured on quartz substrates using a referenced PerkinElmer Lambda 800 UV / Vis spectrometer to clean quartz substrates to nullify absorption losses in the quartz. SnPc gap conductivity was measured for devices consisting of a 1000A thick layer sandwiched between the ITO and Au contacts.

Figure 18 illustrates the JV characteristics of a device with the structure ITO / CuPc (100 A) / SnPc (50 A) / C60 (540 A) / BCP (75 A) / Ag (in this case CuPc indicates copper phthalocyanine, and BCP is batocuproine) in the dark and under various light intensities (Po) of conventional AM1.5G solar lighting. The donor-acceptor heterojunction is defined at the SnPc / C60 i interface and BCP serves as an exciton blocking layer. Adapting the dark current to the classical junction diode theory pn junction diode theory / see J. Xue et al., Applied Physical Letters 84, 3013-3015 (2004)) produces a series resistance of R ^s = 0, 17 Q cm2 and an ideality factor of n = 1.96 ± 0.05. An energy level schematic is shown in the box in Figure 18, where the highest occupied molecular orbital energy levels are measured by ultraviolet photoelectron spectroscopy. The lowest unoccupied molecular orbital energies are estimated using the optical energy gap of each material.

Figures 19 and 20 illustrate various performance parameters of the photovoltaic cell for the same device in function of Po. In this case, FF ~ 0.5 over the entire measured range of intensities, while the open circuit voltage ( V ^oc ) increases logarithmically with Po, with a value of 0.4 V at 1 sol (100 mW / cm2) and up to 0.51 V at 12 soles of intensity. The response capacity (equal to J ^sc / P ^o , where J ^sc is the short-circuit current density) remains relatively constant with Po = (0.05 ± 0.02) A / W. As a result of these trends in device parameters, r¡P increases along with V ^oc at (1.0 ± 0.1)% under conventional AM1.5G solar illumination of 1 sol, reaching (1.3 ± 0.1 )% under 12 soles. The fact that both FF and J ^sc / P ^o are constant with Po from 1 to 12 soles indicates that the collection of carriers in the device is effective, and that the recombination of carriers does not increase significantly with intensity.

The external quantum efficiency (q ^eqe ) and the absorption coefficients ( a) of the active layers of the device are shown in figure 21 as wavelength functions. The low energy Q band of SnPc (dashed line) shows a considerably stronger absorption than that of CuPc (solid line), reaching peak values of a = 3.5 * 105 cm-1 at A = 740 nm, as opposed to 1.3 x 105 cm-1 at A = 620 nm, respectively. Furthermore, a for SnPc extends to NIR, with significant absorption up to A = 1000 nm. The absorption coefficient was calculated from a 50A thick SnPc film; same as the one used in the device. Increased film thickness leads to a change in spectral shape, as the peak at A = 860 nm increases from the peak at 740 nm (data not shown), possibly as a result of molecular aggregates that form SnPc dimers. . That is, if we assume that the thinner SnPc layers do not provide uniform surface coverage, some regions will contain SnPc clusters with a high density of dimers, while other regions will have either a monolayer or no coverage completely absent. dimers. In this case, the added long wavelength peak intensity will also be reduced. The qEQEs of the PV cell in figure 18 (filled circles, figure 21) follow the absorption of the constituent materials, with the contribution of the photocurrent from the C60 generated between 350 <A <550 nm, with a peak qEQE = 36 %, while the SnPc layer contributes from 600 <A <1000 nm, reaching a maximum of 21%.

Optical modeling using transfer matrices (see LAA Pettersson et al., Journal of Applied Physics 86, 487 (1999)) suggests that the SnPc region contributes to an internal quantum efficiency of ~ 40%. There are additional small response peaks in qEQE at A = 575 and 620 nm, corresponding to a contribution from the underlying CuPc layer. This latter response arises as the 50A thick SnPc layer is discontinuous, allowing direct local contact between CuPc and C60 that form efficient charge separation regions, further supporting the assumption of non-layer coverage. uniform for thinner films. (See J. Xue et al., Applied Physical Letters 84, 3013-3015 (2004)).

The central Sn atom is 1.13 A outside the molecular plane, causing SnPc to crystallize in the triclinic phase. (See M.K. Friedel et al., Chemical Communications, 400 (1970); and R. Kubiak et al., Journal of Alloys and Compounds 189, 107 (1992)). X-ray powder diffraction of the purified source material is shown in Figure 22, and the peaks are identified using the known molecular crystal structure. Figure 22 shows a 1000 A thick SnPc film deposited on ITO to have a single peak at 26 = (12.60 ± 0.05) °, corresponding to a lattice plane separation of = (7.03 ± 0.02) A. This indicates that the SnPc film is polycrystalline with preferably flat molecules on the ITO surface, according to previous studies. (See K. Walzer et al., Surf. Sci. 471, 1 (2001); and LB Chen et al., Acta Phys. Sin. 45, 146 (1996)).

Figure 23 shows the surface morphology of a 250A thick SnPc film on quartz. The scanning electron micrograph shows a rough, wavy surface indicative of a polycrystalline film, while the atomic force micrograph (Figure 23, inset) produces an average quadratic roughness of 47 A.

Gap mobility in SnPc films was measured by adjusting the limited space charge current of an ITO / SnPc (1000 A) / Au device to the child's law, (see M. Pope and CE Swenberg, Electronic Processes on Crystals and organic polymers, 2nd ed. (Oxford University Press, New York, 1999); and BP Rand, J. Xue, S. Uchida, and SR Forrest, "Mixed donor-acceptor molecular heterojunctions for photovoltaic application. I. Material properties," presented in the Applied Physics Review on February 9, 2005) assuming a relative dielectric constant of 3 for the organic. This analysis produces a gap mobility of yh = (2 ± 1) * 10-10 cm2 / Vs, compared to the CuPc that has yh = (7 ± 1) * 10-4 cm2 / V ■ s (see BP Rand , J. Xue, S. Uchida and SR Forrest, "Mixed molecular donor receptor heterojunctions for photovoltaic applications. I. Material properties", presented in the Journal of Applied Physics on February 9, 2005).

It is possible that the polycrystalline film of the non-flat SnPc molecule has less molecular orbital overlap compared to CuPc due to greater intermolecular separation. In fact, the separation between the planes formed by the four coordinating N atoms in the SnPc alternates within a unit cell between 3.56 A and 2.87 A (see R. Kubiak et al., Journal of Alloys and Compounds 189 , 107 (1992)), while it is 3.34 A in the case of CuPc (see CJ Brown, J. Chem. Soc. A, 2488 (1968)).

It is the larger of the two separations in the SnPc lattice that determines the mobility of the gap, which may explain the significant differences in yh observed between these materials. The low mobility of the SnPc results in a higher series resistance and, therefore, a low FF in the photovoltaic cells as the layer thickness increases. That is, FF = 0.5 for a 50A thick SnPc layer (compared to Figure 19), dropping to 0.31 for a 200A thick SnPc donor layer at a sun intensity of 1. Placing a 100A thick CuPc wetting layer on the ITO surface prior to SnPc deposition prevents the layer Subsequent C60 acceptor directly contact ITO through discontinuities in the thin donor layer.

In summary, these experiments demonstrate the viability of an organic solar cell with sensitivity extending to the NIR, using SnPc as a thin donor layer of sensitizing electrons. The external quantum efficiency of the cell is significant for wavelengths between 350 <A <1000 nm, remaining above 10% in the range of 350 <A <875 nm. By optimizing the thickness of the layers to retain good charge transport, and by placing the donor / acceptor interface in the region of highest incident light intensity, energy conversion efficiencies of (1.0 ± 0.1) are achieved. % under conventional AM1.5G solar lighting of 1 sun, and up to (1.3 ± 0.1)% under bright lighting. These results highlight the ability of SnPc / C60 binding to find use in tandem organic solar cells (see J. Xue, S. Uchida, BP Rand and SR Forrest, Applied Physical Letters 85, 5757-5759 (2004); and BP Rand, P. Peumans and SR Forrest, Journal of Applied Physics 96, 7519 7526 (2004)) to extend the responsiveness of organic photovoltaic cells to the infrared.

As described above, the organic photosensitive devices of the present invention can be used to generate electrical energy from incident electromagnetic radiation (eg, photovoltaic devices) or they can be used to detect incident electromagnetic radiation (eg, a photodetector or photoconductive cell).

Specific examples of the invention are illustrated and / or described herein. However, it will be appreciated that modifications and variations of the invention are covered by the above teachings and are within the scope of the appended claims.

Claims (14)

1. A photosensitive device comprising: a first electrode and a second electrode; a series of organic photoactive layers arranged between the first electrode and the second electrode, each layer in the series being in direct contact with the next layer in the series, the series of organic photoactive layers being arranged to form at least one donor-receptor heterojunction , comprising the series: a first organic photoactive layer (152) comprising a first host material that serves as a donor; a second organic photoactive layer (980) comprising a second host material disposed between the first organic photoactive layer (152) and a third organic photoactive layer (154); and the third organic photoactive layer (154) comprising a third host material serving as an acceptor; wherein the first host material, the second host material, and the third host material are different, wherein the second organic photoactive layer serves as an acceptor relative to the first organic photoactive layer or as a donor relative to the third photoactive layer, and a distance from any point within the second host material of the second organic photoactive layer to a boundary of that layer is no more than an exciton diffusion length over most of the area of the second organic photoactive layer, and characterized in that the second organic photoactive layer (980) is a unitary layer having openings (1001) therethrough, the first organic photoactive layer (152) being in direct contact with the third organic photoactive layer (154) at through said openings. 2. The photosensitive device of claim 1, wherein said second host material is a small molecule. 3. The photosensitive device of claim 2, wherein said first host material and said third host material are small molecules. The photosensitive device of claim 1, wherein each of the first organic photoactive layer (152), the second organic photoactive layer (980), and the third organic photoactive layer (154) have a different absorption spectrum. 5. The photosensitive device of claim 1, wherein the second organic photoactive layer (980) serves as a donor, and the first host material and the second host material have different absorption spectra. 6. The photosensitive device of claim 5, wherein a band gap of the second host material is less than a band gap of the first host material. 7. The photosensitive device of claim 5, wherein the second host material is selected from tin (II) phthalocyanine (SnPc) and lead phthalocyanine (PbPc). 8. The photosensitive device of claim 7, wherein the third host material is C60. 9. The photosensitive device of claim 1, wherein the second organic photoactive layer (980) serves as an acceptor, and the second host material and the third host material have different absorption spectra. The photosensitive device of claim 9, wherein a band gap of the second host material is less than a band gap of the third host material. 11. The photosensitive device of claim 1, wherein the thickness of the second organic photoconductive layer (980) is not greater than 200 A. 12. The photosensitive device of claim 1, wherein the thickness of the second organic photoconductive layer (980) is not greater than 100 A. The photosensitive device of claim 1, wherein the series of organic photoactive layers further comprises a crude or mixed acceptor-donor layer disposed between the first organic photoactive layer and the third organic photoactive layer, said acceptor-donor layer including raw or mixed both the first host material of the first organic photoactive layer and the third host material of the third organic photoactive layer. The photosensitive device of claim 1, wherein the distance from any point within the second host material of the second organic photoactive layer (980) to a boundary of that layer is no more than an exciton diffusion length at throughout the entire area of the second organic photoactive layer.